Proximity Detection

ABSTRACT

A method of estimating the proximity of a first device to a second device in a network causes the first device to perform a first measurement in a first period during which a control message broadcasted over the network instructs devices in the network to not transmit during the first period, wherein the second device disregards the control message and transmits a first signal during the first period which is measured by the first device during the first period, and forms a measure of the proximity of the first device to the second device in dependence on a strength of the first signal.

BACKGROUND OF THE INVENTION

This invention relates to methods, apparatus and systems for estimatingthe proximity of a wireless device to one or more other wirelessdevices.

The ability to communicate data wirelessly at high data rates has led tomany new and improved applications and devices. Some systems whichtraditionally were wired are now being improved by replacing the wireswith wireless capabilities. For example, traditional 5.1 surround soundsystems require 6 speakers to be located in different parts of a roomand to be wired to a central receiver. Many users have found the cablingrequired to install such a system to be very inconvenient andcumbersome. Thus multi-speaker systems have been provided with wirelesscapability which allows users to easily install and use the systems.

Some wireless multi-speaker systems employ a hub which is wirelesslyconnected to the speakers in the system. The hub can store a user'smusic collection and can control the output of the speakers in thesystem. A user can select the output of the speakers via, for example, auser interface on the hub or a device connected to the hub. In amulti-room environment, for example, a speaker may be provided in eachroom in a house. A user can select, via the hub, which speaker is toprovide the audio output and the hub transmits audio data to theselected speaker for playback. This allows the user to listen to audioin whichever room a speaker is placed without requiring each room tohave an individual store of music.

Typically such systems employing a hub operate in a proprietarypeer-to-peer mesh network. Such a proprietary system provides the userwith flexibility, control and freedom to implement different functionswith their proprietary devices. However, a problem with such proprietarysystems is that they may not be compatible with devices from othervendors. This can restrict other vendors from making devices (such asadditional speakers or media sources) for use in the wireless speakersystem and thus also restricts consumer choice.

There is therefore a need for techniques that allow devices to haveadditional functionality whilst maintaining compatibility with otherdevices in an environment such as a wireless multi-room environment.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof estimating the proximity of a first device to a second device in anetwork of devices comprising at least the first and second device, eachdevice in the network being capable of communicating according to apredetermined wireless communications protocol, the method comprising:analysing at the first device a first control message which according tothe communications protocol is configured to cause the first device toperform a first channel quality measurement during a first period oftime; broadcasting a second control message over the network whichaccording to the communications protocol instructs devices connected tothe network to not transmit during the first period of time to enablethe first channel quality measurement to be performed; at the seconddevice, disregarding said instruction and transmitting a first signalduring at least a portion of the first period of time; at the firstdevice, performing the first channel quality measurement in response tothe first control message during the first period of time; and forming ameasure of the proximity of the first device to the second device independence on a strength of the first signal received at the firstdevice.

The method may further comprise the step of: at the second device,sending the first control message. The second device may broadcast thesecond control message.

The network of devices may comprise a third device, and the method mayfurther comprise the step of: at the third device, sending the firstcontrol message. The method may further comprise the step of: at thethird device, sending the second device a transmit-time message, thetransmit-time message configured to cause the second device to performsaid transmitting step.

The first control message may be generated at the first device.

Preferably, the transmission power of the first signal is modulated in apredefined pattern. Preferably, said forming step comprises analysing afirst report message so as to detect a representation of the predefinedpattern, said first report message comprising a histogram indicatingreceived power at the first device.

Preferably, the method further comprises the step of: analysing a firstreport message which according to the first communication protocolindicates the results of the first channel quality measurement.

Preferably, the method further comprises the step of: at a or the thirddevice, transmitting a second signal during at least a portion of asecond period of time. The first and second signals may be transmittedat substantially the same power. The said forming step may additionallybe dependent on a strength of the second signal received at the firstdevice. The said forming step may comprise comparing the strength of thefirst signal to the strength of the second signal so as to determinewhich of the second and third devices is nearest to the first device.The method may further comprise the steps of: at the first device,selecting media for playback; selecting said determined second or thirddevice; and playing back said selected media at said selected second orthird device.

Preferably, the method further comprises the steps of: broadcasting athird control message which according to the communications protocol iscapable of causing devices to not transmit during a third period oftime; and at the first device, performing a second channel qualitymeasurement during the third period of time, wherein said forming stepis additionally dependent on the second channel quality measurement.

Preferably, the predetermined wireless communications protocol is a IEEE802.11 protocol. Preferably, the first control message comprises ameasurement request element as defined by the IEEE 802.11 protocol.

According to a second aspect of the invention there is provided awireless device for estimating the proximity of a first device to asecond device in a network of devices comprising at least the first andsecond devices, each device in the network being capable ofcommunicating according to a predetermined wireless communicationsprotocol, the wireless device comprising: a controller configured togenerate a first control message which according to the communicationsprotocol is configured to cause the second device to perform a firstchannel quality measurement during a first period of time; a transceiverconfigured to broadcast a second control message over the network whichaccording to the communications protocol is configured to instructdevices connected to the network to not transmit during the first periodof time to enable the first channel quality measurement to be performed,the controller being further configured to cause the first device todisregard said instruction and transmit a first signal during at least aportion of the first period of time; and a proximity estimatorconfigured to form a measure of the proximity of the first device to thesecond device in dependence on a strength of the first signal receivedat the second device.

The wireless device may be the first device, and the transceiver may befurther configured to: send the first control message to the seconddevice; and receive a first report message from the second device.Preferably, the first report indicates the results of the first channelquality measurement in accordance with the communications protocol.

The controller may be configured to generate a transmit-time message;and the transceiver may be configured to send the transmit-time messageto the first device so as to cause the first device to disregard saidinstruction and transmit the first signal.

The wireless device may be the second device.

The transceiver may be further configured to: send the first controlmessage to the second device; and receive a first report message fromthe second device which according to the communications protocolindicates the results of the first channel quality measurement.

According to a third aspect of the invention there is provided a methodof estimating the proximity of a first device to a second device in anetwork of devices comprising at least the first and second device, eachdevice in the network being capable of communicating according to apredetermined wireless communications protocol, the method comprising:selecting a first channel for performing a first channel qualitymeasurement, the first channel being a channel that is not in use by thenetwork of devices; receiving at the first device a first controlmessage which according to the communications protocol is configured tocause the first device to perform the first channel quality measurementon the first channel during a first period of time; at the seconddevice, transmitting a first signal on the first channel during at leasta portion of the first period of time; at the first device, performingthe first channel quality measurement in response to the first controlmessage during the first period of time; and forming a measure of theproximity of the first device to the second device in dependence on astrength of the first signal received at the first device.

The method may further comprise the step of maintaining a connectionbetween the first and second device on a second channel during at leasta portion of the first time period.

The method may further comprise the step of maintaining a connectionbetween the second device and a third device on a second channel duringat least a portion of the first time period.

The network of devices may comprise a third device, and the method mayfurther comprise the steps of: at the third device, sending the firstcontrol message; at the third device, sending the second device atransmit-time message, the transmit-time message configured to cause thesecond device to perform said transmitting step.

The method may further comprise the step of: at the second device, priorto said transmitting step, disabling a connection between the seconddevice and the first device, said connection being over a secondchannel.

The network of devices may comprise a fourth device, the method mayfurther comprise the step of maintaining a connection between the thirdand fourth device on a second channel during at least a portion of thefirst time period.

According to a fourth aspect of the invention there is provided awireless device for estimating the proximity of a first device to asecond device in a network of devices comprising at least the first andsecond devices, each device in the network being configured tocommunicate over a communications channel according to a predeterminedwireless communications protocol, the wireless device comprising: acontroller configured to select a first channel for performing a firstchannel quality measurement, the first channel being a channel that isnot the communications channel; a transceiver configured to send a firstcontrol message to the second device which according to thecommunications protocol is configured to cause the second device toperform the first channel quality measurement on the first channelduring a first period of time, the controller being further configuredto cause the first device to transmit a first signal during at least aportion of the first period of time; and a proximity estimatorconfigured to form a measure of the proximity of the first device to thesecond device in dependence on a strength of the first signal receivedat the second device.

The first device may be the wireless device, and the transceiver may befurther configured to: send the first control message to the seconddevice; and receive a first report message from the second device.Preferably, the first report indicates the results of the first channelquality measurement in accordance with the communications protocol.

The controller may be configured to generate a transmit-time message;and the transceiver may be configured to send the transmit-time messageto the first device so as to cause the first device to disregard saidinstruction and transmit the first signal.

According to a fifth aspect of the invention there is provided machinereadable code for implementing the methods.

According to a sixth aspect of the invention there is provided a machinereadable storage medium having encoded thereon non-transitory machinereadable code for implementing the methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 depicts an example network comprising wireless devices;

FIG. 2 depicts an example process for estimating the proximity of awireless device to other wireless devices;

FIG. 3 depicts another example process for estimating the proximity of awireless device to other wireless devices;

FIGS. 4a and 4b depicts yet another example process for estimating theproximity of a wireless device to other wireless devices;

FIG. 5 is a schematic diagram of an access point;

FIG. 6 is a schematic diagram of a wireless station;

FIG. 7 is a chart showing an example power transmission pattern for aninterfering signal;

FIGS. 8a and 8b are example histograms from a received power indicator(RPI) report;

FIG. 9a is a chart showing an example power transmission pattern for aninterfering signal;

FIG. 9b is a chart showing measured received power at differentdistances; and

FIGS. 10a to 10d are example RPI histograms for the measured receivedpower at the different distances of FIG. 9 b.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application. Various modifications to the disclosedembodiments will be readily apparent to those skilled in the art.

The general principles defined herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features disclosed herein.

The Wi-Fi communications protocol is commonly used for many wirelessapplications between computing devices, network servers and the like. Itis increasingly being used in domestic applications for functions suchas streaming or playing back audio and/or video data and othermulti-media applications such as gaming using portable gaming devices.The methods, devices and networks disclosed below will be described withreference to devices and networks that can operate in accordance withthe IEEE 802.11 Wi-Fi communications protocol. The general principlesdescribed below can be applied to devices and systems operatingaccording to other communications protocols such as Bluetooth.

FIG. 1 is a schematic diagram illustrating an example network 100, whichmay be, for example, implemented in a home having an access point (AP)10 and multiple wireless stations (STAs) 11, 12. In one example, theSTAs 11, 12 may be media playback devices such as wireless speakers. TheAP 10 may be connected to a media source and may also include aloudspeaker for playback. The media source can be located in theinternet (e.g. a streaming service), connected to the AP 10 (e.g. storedon a local computer, mobile device, or network drive), or can be derivedfrom one of the STAs (e.g. the media may be broadcast via DAB andreceived at one STA equipped with a DAB receiver and provided to anotherSTA over Wi-Fi) or stored on an STA. A user may operate a portabledevice 13, which may be, for example, a smartphone, tablet, laptop,watch, etc that is able connect, via Wi-Fi, to the AP 10 or, in somemodes (such as Wi-Fi direct), to one of the STAs 11 or 12 to controloperation thereof.

In some applications, it may be advantageous to determine the positionof the portable device 13 relative to the AP 10 and STAs 11, 12. Forexample, in a multi-room media environment, it may be advantageous todetermine the position of a user (who may be carrying the portabledevice 13) relative to wireless speakers, which may be AP 10 and STAs11, 12. By determining the position of the portable device 13, the AP10, or STA 11 or 12 that is closest to the portable device 13 can beautomatically selected, e.g., for playback of audio.

The proximity of the portable device 13 to the AP 10 or STAs 11, 12 canbe estimated by manipulating certain features of the Wi-Fi standard,without breaking the rules laid out by the standard. According to thestandard these features are used for purposes other than determining theproximity of a Wi-Fi device relative to other Wi-Fi devices. Byutilising these features in a new way, it is possible for devices toconform with and operate in accordance with the standard and still beable to provide the new feature of proximity estimation that is notdefined in the standard.

FIG. 2 depicts a process for estimating the proximity of the portabledevice 13 to the AP 10 and STA 11. The process for estimating theproximity of the portable device 13 to STA 12 is similar to that for STA11.

In the example processes described herein, the wireless network 100 canoperate in infrastructure mode or in an ad-hoc or independent basicservice set (IBSS) mode. In the IBSS mode, the AP 10 may be an STA thatoperates as a software enabled access point (softAP). The softAP canconnect to another AP (not shown) to enable data to be sent to and fromthe IBSS.

The AP 10 sends a first control message to the portable device 13. Thefirst control message may be a message that is compliant with the Wi-Fistandard, such as a measurement request 201. According to the standard,such measurement requests are used to avoid co-channel operation withinterfering systems (such as radar systems), gathering information onthe state of a channel so as to assist in the choice of a new channel,and assess the general level of interference present on a channel. Thus,such measurement requests are traditionally used by Wi-Fi compliantdevices to detect interference so as to avoid channels with highinterference.

As required by the Wi-Fi standard, the measurement request 201 containsa request that the receiving STA performs the specified measurementaction. The AP 10 is able to specify certain parameters for themeasurement request 201, such as the measurement type (which may be abasic request, clear channel assessment (CCA) request or receive powerindicator (RPI) histogram request), channel number, measurement starttime and measurement duration. Thus the AP 10 is able to specify aperiod of time t1 for the portable device 13 to perform the measurement.

The AP 10 broadcasts a second control message (which may have adifferent format to the first control message) which can cause devicesthat receive the message to remain quiet (i.e. not transmit) during aspecified period of time. The control message may be a frame (such as abeacon frame or probe response frame) that contains a quiet element(hereinafter referred to as quiet message 202). As specified by theWi-Fi standard, transmitting one or more quiet elements allows thescheduling of quiet intervals, during which no transmission on thenetwork 100 shall occur in the current channel. Fields within the quietelement such as the quiet count, quiet period, quiet duration and quietoffset can be used by the AP 10 to specify a period of time during whichdevices in the network are to remain quiet. The AP 10 broadcasts thequiet message 202, which can be received by the devices in the network(e.g. STAs 11 and 12 and portable device 13) to instruct those devicesto remain quiet during specified period of time t1. By broadcasting thequiet message 202, the AP 10 also instructs itself to remain quietduring specified period of time t1. The specified period of time in thequiet message can be the same time period t1 that is specified in themeasurement request 201. This allows the portable device 13 to performthe measurement without interference from STAs 11 and 12 as they willnot transmit during quiet intervals 205 and 206.

The portable device 13 performs the measurement specified by themeasurement request 201 during time period t1. During at least a portionof time period t1, the AP 10 transmits a first signal 204. By generatingand broadcasting the quiet message 202 the AP 10 is also required toremain quiet. However, the AP 10 disregards the instruction to remainquiet and does not stay quiet during the quiet interval so that it cantransmit first signal 204. The first signal 204 can be detectable by theportable device 13 while performing the measurement (if the portabledevice 13 is within range at that time). The first signal 204 will betransmitted on the channel (which can be specified in the measurementrequest 201) on which the measurement 203 is being performed. Thetransmission power of the first signal 204 may be at a predeterminedpower level.

The first signal 204 can be transmitted during the whole or a portion ofthe time period t1. For example, the first signal 204 may be transmittedduring half of time period t1. By varying the power of the first signal204 in a predetermined pattern, it can be possible to determine that asignal measured during the measurement 203 is the first signal 204. Thesub-section entitled “Detecting a predefined power pattern” belowprovides examples of how the first signal 204 can be identified.

After time period t1 expires, the measurement process 203, transmissionof the first signal 204 and quiet interval 205 and 206 will have ended.

In accordance with the Wi-Fi standards, in response to the measurementrequest 201, the portable device 13 transmits a measurement report 207which provides measurement information gathered during the measurementprocess 203. The measurement report 207 can be received by the AP 10. Independence on the measurement report 207, the AP 10 can form a measureof the proximity of the portable device 13 to the AP 10. The measurementinformation provided by the measurement report 207 may be indicative ofthe first signal 204 transmitted by the AP 10. As the other STAs 11 and12 are quiet while the measurement 203 is taking place, the receivedpower at the portable device 13 may be mostly from the first signal 204and any background noise (which can be subtracted as described below).Thus the information provided in the measurement report 207 can beindicative of the received power of first signal 204 (and not anytransmissions from STAs 11 and 12), which can be used to estimate theproximity of the portable device 13 to AP 10.

The measurement report 207 may comprise a RPI histogram report, asdefined in the Wi-Fi standard. The RPI histogram report comprises RPIdensities observed during the measurement period 203 at eight RPI levelsor “buckets”. This can provide an indication of the received power levelof the first signal 204 at the portable device 13, which can provide anindication of the proximity of the portable device 13 to the AP 10. Forexample, if the RPI histogram report has a greater number of RPIdensities at RPI 0 (Power≤−87 dBm), than at higher RPI densities, thenthis can indicate the portable device 13 is relatively far from the AP10. If the RPI histogram report has a greater number of RPI densities atRPI 7 (−57 dBm<Power), than at lower RPI densities, then this canindicate that the portable device 13 is relatively close to the AP 10.Thus a measure of the distance between the AP 10 and the portable device13 can be formed by analysing the RPI histogram report. Examples of howthe RPI histogram report may be utilised to provide an indication of theproximity of devices are given below in the “Detecting a predefinedpower pattern” sub-section.

The AP 10 can also estimate the proximity of portable device 13 to STA11. Steps 208 to 215 described below describe a process for estimatingthe proximity of the portable device 13 to STA 11. These steps may beperformed independently of the proximity measure of the portable device13 to AP 10 described above in steps 201-207. Alternatively, steps 208to 215 can be performed in addition to the proximity measure of theportable device 13 to AP 10.

The AP 10 sends a second measurement request 208 to the portable device13. Measurement request 208 may specify the same parameters asmeasurement request 201, but with a different measurement start time.Thus the AP 10 is able to specify a second period of time t2 for theportable device 13 to perform a second measurement 211. The AP 10broadcasts a second quiet message 209, instructing devices in thenetwork 100 (including the AP 10 itself) to remain quiet during timeperiod t2.

The AP 10 sends a message (herein after referred to as transmit-timemessage 210) to STA 11 that instructs STA 11 to transmit a second signal212 during at least a portion of time period t2. The transmit-timemessage may be sent before or after the AP broadcasts the second quietmessage 209 (but before the second measurement 211). The transmit-timemessage 210 may indicate parameters for the second signal 212 so thatthe STA 11 transmits a second signal 212 that has substantially similarproperties (such as the transmit power, the power variation pattern,channel, etc) as the first signal 204. The properties of the first andsecond signals 204 and 212 can be predetermined such that, by default,the AP 10 and STAs 11 and 12, transmit the signals with the samepredetermined properties. When estimating the proximity of the portabledevice 13 to the STA 11, it is preferable that the first and secondsignals 204 and 212 have similar properties. This allows the measurementresults from those signals to be compared to determine the proximity ofthe portable device 13 relative to the AP 10 and the STA 11. The AP 10will remain quiet (i.e. not transmit) during time period t2. This quietinterval 213 for the AP 10 may be triggered by sending quiet message 209or by sending the transmit-time message 210.

STA 11 receives the transmit-time message 210, which causes the STA 11to disregard (or, in other words, override) the instruction in the quietmessage 209 to remain quiet and transmit the second signal 212 duringthe second time period t2.

The transmit-time message 210 may be a message that is sent at a higherlevel than a Wi-Fi command. For example, the transmit time message 210may be a message sent over a layer (e.g. a UPNP command layer or anapplication layer) that is above the lower level Wi-Fi functions andallows the devices in the network to exchange data.

During the second time period t2, the portable device 13 performs thesecond measurement 211, STA 11 transmits the second signal 212 and AP 10and STA 12 do not transmit during quiet intervals 213 and 214. Thus, theportable device 13 is able to receive a second signal 212 from STA 11without interference from the AP 11 and STA 12. In accordance with theWi-Fi standard, a second measurement report 215 is sent from theportable device 13 to the AP 10 indicating the received power of thesecond signal 212 from STA 11. A measure of the proximity of theportable device 13 to STA 12 may be, for example, based on the secondmeasurement report alone if, for example, the second measurement reportcomprised a RPI histogram report, as described above. Alternatively, themeasure of proximity could be based on a comparison of the first andsecond measurement reports. The received power levels indicated in thefirst and second measurement reports 207 and 215 can be compared so asto estimate the proximities of the AP 10 and STA 11 relative to theportable device 13. For example, if the reports 207 and 215 indicatethat the power received during the second measurement 211 is greaterthan the power received during the first measurement 203, the AP 10 maydetermine that STA 11 is closer to the portable device 13 than the AP10.

In the processes described herein, the AP 10 can determine backgroundnoise power by sending additional measurement requests to the portabledevice 13 to perform background measurements. The AP 10 may broadcastadditional quiet messages so that the devices in the network (includingthe AP 10) are quiet during the background measurements. These quietmessages are similar to quiet messages 202 and 209, however they willnot be disregarded as all of the devices in the network will need toremain quiet for the background measurements to take place. The detectedpower levels from the background measurements can be subtracted from thepower levels received during the measurement processes 203 and 211 toprovide a more accurate measure of the received power from the first andsecond signals 204 and 212.

The proximity of STA 12 (and any other STAs that may be connected to theAP 10) to the portable device 13 may be estimated in a similar manner tothat of STA 11. The proximity estimate may be performed independently ofthe other measurement reports indicating the received power from theother devices in the network. Alternatively, the proximity estimate forSTA 12 may be performed based on one or more additional measurementreport 207 and/or 215 indicating the received power from AP 10 and/orSTA 11.

The AP 10 can receive a plurality of measurement reports from theportable device 13, each report indicating the received power level of asignal transmitted by a device in the network (AP 10 and STAs 11, 12).The level of power in each report provides an indication of the distanceof the transmitting device from the measuring device, which in this caseis the portable device 13. By comparing the power levels of each reportit is possible to determine the relative proximity of the transmittingdevices from the portable device 13.

The estimate of the proximity of the portable device 13 to the AP 10 andSTAs 11 and 12 may be used to select one of the AP 10, STA 11 or STA 12that is closest to the portable device 13. This selection of the closestdevice may be, for example, a selection of the closest wireless speakerto the smartphone of a user. The proximity estimate may be carried outperiodically for all the devices that are connected to the AP 10 as theportable device 13 may move to different locations and thus the closetdevice 10, 11 or 12 to the portable device 13 may change. Thus in amulti-room environment, a user carrying a portable device 13 such as asmart phone may move from a first room to a second room and have thespeaker in the second room automatically play the music that was beingplayed in the first room without the user having to select the speakerin the second room. The AP 10 can determine which speaker is closest tothe user, select the speaker that is closest to the user, and send amessage to that closest speaker to play music. If a speaker that isplaying music is not the speaker that is closest to the user, the AP 10may also send a message to that speaker to stop playing music.

Sequences of proximity measurements can be stored and analysed in anapplication layer program at the AP, for example to determine whetherthe portable device 13 is fixed or mobile, or to track the movementrelative to one or more of devices 10, 11 or 12. This can be used topredict movement (e.g. moving away from or towards given a given device10, 11 or 12), or can be correlated with time-of-day information togenerate typical movement patterns for a given user of a portable device13 (e.g. certain times of the day spent in particular rooms).

In the process described above, the steps (e.g. receiving a measurementrequest, performing a measurement in response and in accordance withthat request, sending a measurement report, receiving quiet messages,etc) carried out by the portable device 13 can be carried out by anydevice that conforms with the Wi-Fi standard. Also, some of the stepscarried out by the STAs can be carried out by any device that conformswith the Wi-Fi standard (e.g. receiving a quiet message and nottransmitting in response to the quiet message).

Some of the steps carried out by the AP 10 and STAs 11 and 12 may notconform to the Wi-Fi standard (e.g. disregarding the second quietmessage 209 to transmit the second signal 212 during the time period t2as the quiet interval specified in the received second quiet message209). In this case, the STA 11 receives a transmit-time message 210(which can be communicated, for example, in accordance with a higherlevel protocol, such as UPnP), which causes the STA 11 to override anyactions set out by the Wi-Fi controller of that STA 11 and insteadtransmit the second signal 212 as specified by the transmit-time message210.

The overriding action described above is possible by allowing layersabove the Wi-Fi MAC layer (e.g. applications) to control certain aspectsof Wi-Fi MAC layer that would normally not be permissible under theWi-Fi standard.

FIG. 3 depicts another process for estimating the proximity of theportable device 13 to the AP 10 and STA 11. In this process, certainaspects of the Wi-Fi MAC layer at the portable device 13 are capable ofbeing controlled by higher layers, such as applications.

Steps 301 to 308 describe the process for estimating the proximity ofthe portable device 13 to the AP 10. Steps 309 to 316 describes anotherprocess for estimating the proximity of the portable device 13 to STA11. The process for estimating the proximity of the portable device 13to STA 11 may be performed independently of the process for estimatingthe proximity of the portable device 13 to AP 10, as described in steps301 to 308. Alternatively, steps 309 to 316 can be performed in additionto steps 301 to 308 to estimate the proximity of the portable device 13to AP 10 and/or STA 11.

A higher level entity, such as an application, at the AP 10 can triggerthe sending of a control message to the lower MAC layers of the AP 10.For example, the control message 301 may be a message that is compliantwith the Wi-Fi standard, such as a MLME-MEASURE.request message. TheMLME-MEASURE.request message causes the AP 10 to perform measurements ina similar manner to the measurement requests 201 and 208 describedabove. The control message 301 is able to specify a period of time t3for the AP to perform a measurement 305. Similarly to above, the AP 10broadcasts a quiet message 302 specifying that devices in the network100 remain quiet during time period t3.

The AP 10 sends a transmit-time message 303 (similarly to transmit-timemessage 210 described above) to portable device 13, which instructsportable device 13 to transmit a signal 304 during at least a portion oftime period t3. The signal 304 may be similar to signals 204 and 212described above. The transmit-time message 303 causes the portabledevice 13 to disregard the quiet message 302 and transmit the signal 304during time period t3. The transmit-time message 303 may be sent beforeor after quiet message 302 (but before the measurement period 305).

During the time period t3, AP 10 performs the measurement 305 asrequested, portable device 13 transmits the signal 304 and STAs 11 and12 remain quiet during quiet intervals 306 and 307. Thus, the AP 10 isable to receive signal 304 from portable device 13 without any potentialinterference from STAs 11 and 12.

The AP 10 may generate measurement report 308 (which may be similar tomeasurement reports 207 and 215 described above) indicating the receivedpower of the signal 304 from the portable device 13. The measurementreport 308 may reported to the higher layers entitles at the AP 10 via,for example, the MLME-MEASURE.confirm primitive in accordance with theWi-Fi standard.

Steps 309 to 316 describe a process for estimating the proximity of theportable device 13 to STA 11.

The AP 10 sends a measurement request 309 to STA 11. The measurementrequest 309 is similar to measurement requests 201 and 208 describedabove. Similarly to above, the AP 10 broadcasts a quiet message 310specifying that devices in the network 100 remain quiet during timeperiod t4.

The AP 10 sends a transmit-time message 311 (which is similar totransmit-time message 210 described above) to portable device 13, whichinstructs portable device 13 to transmit a signal 312 during at least aportion of time period t4. The signal 312 may be similar to signals 204and 212 described above. The transmit-time message 311 causes theportable device 13 to disregard the quiet message 310 and transmit thesignal 312 during time period t4. The transmit-time message 311 may besent before or after quiet message 310 (but before the measurementperiod 313).

During time period t4, STA 11 performs measurement 313 as requested,portable device 13 transmits signal 312 and AP 10 and STA 12 remainquiet during quiet intervals 314 and 315. Thus, the STA 11 is able toreceive signal 312 from portable device 13 without any potentialinterference from AP 10 and STA 12.

The STA 11 generates and sends measurement report 316 (which may besimilar to measurement reports 207 and 215 described above) to the AP 10indicating the received power of signal 312 from the portable device 13.As described above, the measurement report 316 can be used to determinea measure of the proximity of the portable device 13 to STA 11.Alternatively, a measure of proximity of the portable device 13 to STA11 could be based on a comparison of measurement reports 308 and 316.

FIG. 4a depicts another process for estimating the proximity of theportable device 13 to the AP 10. FIG. 4b depicts another process forestimating the proximity of the portable device 13 to the STA 11. Inthese processes, the signal that is measured during the measurementperiod is transmitted on a channel that is not currently being used bythe devices in the network. Thus, a channel that is different to thechannel being used to communicate data between the devices can be usedto carry out the measurements. In these examples, a quiet message neednot be sent and so the devices in the network can continue tocommunicate data without having to observe a quiet period. This can beparticularly advantageous when streaming real-time media betweendevices.

In the processes of FIGS. 4a and 4b , the AP 10 can determine a channelupon which the measurement for estimating proximity should be carriedout. This determination can be based, for example, on a channelassessment (e.g. a CCA) carried out by the AP 10 or another device onthe network to select a clear channel or a random selection of a channel(hereinafter referred to as the “selected channel”) that is notcurrently being used within the network. Preferably, the selectedchannel does not overlap with a channel that is currently being used bydevices in the network so that normal traffic does not interfere withthe proximity measurement. The channel currently being used (hereinafterreferred to as the “connection channel”) for traffic may have beendetermined, for example, when connections between the devices in thenetwork were established or during a channel switch.

As shown in the process in FIG. 4a , the AP 10 sends a measurementrequest 601 to the portable device 13. The measurement request 601 maybe similar to measurement request 201 described above. The AP 10specifies the selected channel number in the measurement request 601 anda time period for carrying out the measurement. The measurement request601 may be sent over the connection channel between the AP 10 and theportable device 13. The specified selected channel in the measurementrequest 601 is different to the connection channel.

The AP 10 may have two RF interfaces (e.g. in the case of WiFi devices,the 802.11n or later standards provide for 2.4 GHz and 5 GHzinterfaces), which can allow the AP 10 to communicate simultaneously ontwo different channels. Thus, at the measurement time period, the AP 10transmits a signal 602 (which may be may be similar to signal 204described herein) on the selected channel whilst maintaining aconnection with STA 11 and/or STA 12 on the connection channel, overwhich it can communicate data 603 and/or 604 with STA 11 and/or STA 12respectively. Once the measurement of signal 602 on the selected channelis complete, a measurement report 605 (which may be similar tomeasurement report 207 described herein) for that signal 602 isgenerated by the portable device 13. The portable device 13 may thentransmit the measurement report 605 to the AP 10 over the connectionchannel. The measurement report 605 may then be used to estimate theproximity of the portable device 13 to the AP 10 as described herein inrelation to measurement report 207.

The process of FIG. 4b shows how the proximity of the portable device 13to STA 11 can be estimated. The proximity of portable device 13 to STA12 can also be estimated in a similar manner. The AP 10 sends ameasurement request 601, as described above, to the portable device 13over the connection channel. The measurement request specifies themeasurement time period and the selected channel for carrying out themeasurement. The AP 10 also sends a transmit-time message 606 (which maybe similar to transmit-time message 210 described herein) to STA 11specifying the time period and channel to transmit signal 607 (which maybe similar to signal 602 described above).

STA 11 may have one or two RF interfaces. If STA 11 has two RFinterfaces, then STA 11 can transmit signal 607 on the selected channeland simultaneously maintain a connection with AP 10 on the connectionchannel, as described above in relation to the AP 10 with two RFinterfaces. If STA 11 has one RF interface, then the STA 11 maytemporally disable its connection with AP 10 and switch channels fromthe connection channel to the selected channel to carry out thetransmission of signal 607. Whilst signal 607 is being transmitted bySTA 11, AP 10 may maintain a connection with STA 12 over the connectionchannel to communicate data 608. Once signal 607 has been transmitted,STA 11 can switch back to the connection channel and reconnect with theAP 10 to transmit measurement report 609, which may then be used toestimate the proximity of the portable device 13 to the STA 11 asdescribed herein in relation to measurement report 207. Thus, theprocesses described in relation to FIGS. 4a and 4b allow the proximityestimate to be carried out without any quiet periods.

FIG. 5 is a schematic diagram of AP 10. The AP 10 may be a dedicatedaccess point device or an STA operating as a softAP. The AP 10 comprisesa Wi-Fi antenna 401, a Wi-Fi transceiver 402, a Wi-Fi driver 403 and aproximity measurer 404. The Wi-Fi antenna 401, Wi-Fi transceiver 402 andWi-Fi driver 403 may operate in accordance with the Wi-Fi standard.Proximity measurer 404 may be an application that controls the processfor estimating the proximity of the portable device 13 to the AP 10and/or STAs 11 and 12 in the network 100. The proximity measurer 404 canhave access to the Wi-Fi driver 403, which allows it to trigger orcontrol certain functions of the Wi-Fi driver 403. These functions maybe to, for example, populate and trigger sending measurement requestsand quiet messages, accessing received measurement reports, cause thetransmission of the signal to be measured, override any quiet messages,etc. The proximity measurer 404 may collate and analyse the data fromreceived measurement reports to form a measure of the proximity of theportable device 13 to the AP 10 and/or STAs 11 and 12 in the network, asdescribed above.

When sending the measurement request messages to a device for estimatingproximity, the AP 10 can store a record of the measurement start timeagainst the identity of the device that is making the measurement. Thereceived measurement report from that device contains the start time ofthe measurement so the AP 10 can correlate the measurement reportreceived to the device that made the measurement. This allows the AP 10to extract the correct measurement report for the device from any otherdevices that may be making interference measurements normally (as partof the standard).

In the example of a wireless media device, the proximity measurer 404may determine which of the AP 10 or STAs 11 or 12 is closest to theportable device 13 and provide the identification of the closest deviceto media controller 405. The media controller 405 controls the playbackof media and can select the identified closest device for playback ofmedia.

FIG. 6 is a schematic diagram of STA 11, 12. The STA 11, 12 may or maynot have any AP or softAP capabilities. The STA 11, 12 comprises a Wi-Fiantenna 501, a Wi-Fi transceiver 502, a Wi-Fi driver 503 and acontroller 504. The Wi-Fi antenna 501, Wi-Fi transceiver 502 and Wi-Fidriver 503 may operate in accordance with the Wi-Fi standard.

In the proximity estimation process described above in FIG. 2, the STA11 is required to disregard quiet message 209 and transmit a signal 212during a quiet interval t2. Reception of the quiet message 209 wouldnormally cause the Wi-Fi driver 503 to cause the Wi-Fi transceiver 502to not transmit during the quiet period t2. However, controller 504,which may be an application running on the STA 11 or 12, receivesinstructions (prior or subsequent to receiving the quiet message 209)via the transmit-time message 210 to transmit signal 212 during t2. Thecontroller 504 can access the Wi-Fi driver 503 and cause it to overridethe quiet interval and transmit signal 212.

In the proximity estimation process described above in FIG. 3, STAs 11and 12 are not required to disregard quiet messages. Thus, in this case,the STAs 11 and 12 need not comprise controller 504. However, in thiscase, the portable device 13 would need to comprise such as controllerto enable it to disregard quiet messages 302 and 310 so that it cantransmit signals 304 and 312 during quiet intervals t3 and t4.

The AP 10 and STAs 11 and 12 may comprise a media output (406 and 506respectively) for playing back media received at the Wi-Fi antenna (e.g.from portable device 13). The media outputs 406 and 506 may be, forexample, an audio and/or visual output such as a speaker and/or display.The AP 10 and STAs 11 and 12 may also comprise one or more media inputs(not shown) which provide a connection to a media source (not shown) viaa communication means other than Wi-Fi. For example, the media input maybe a 3.5 mm input for auxiliary devices, a USB port, an Ethernet port, aconnection to a DAB receiver, a Bluetooth transceiver, etc. The mediainputted at the media input may outputted by media output 406 or 506 orsent over Wi-Fi to another device in the network 100 that has beenselected for playing back media by the media controller 405.

In an example of a multi-speaker environment, AP 10 and STAs 11 and 12may be wireless speakers. The AP 10 may be a softAP and STAs 11 and 12may also have the capability to be a softAP. One of devices 10, 11 or 12may be selected as a master speaker, which controls the operation ofslave speakers. Preferably, the master speaker will be selected tooperate as the soft AP. Thus the device 10, 11 or 12 that operates asthe softAP can be changed.

Detecting a Predefined Power Pattern

As mentioned above, the measurement report 207, 215, 308, 316, 605and/or 609 may comprise a RPI histogram report. The RPI histogram reportis based on received interference power, such that samples of thereceived power are taken during a sampling time period, and the receivedpower over the sample period is categorized as falling into one power“bucket”. There are 8 buckets in the Wi-Fi standard. The number ofsamples falling into each bucket are counted to generate the histogram.The RPI histogram report can be utilised to estimate the proximity oftransmitting or “interfering” devices to devices carrying out the RPImeasurement.

In this example, the device transmits signals 204, 212, 304, 312, 602and/or 607 (hereinafter referred to as the “interferer” device) in astepped interference pattern. The interferer initially transmits at afirst power level for a known time period, and then switches to adifferent power level. This generates two distinct spikes in thehistogram of the RPI histogram report. The difference between the powerlevels controls the spacing between the spikes in the histogram. Byfinding the spikes in the histogram, a distance measure can be derivedfor comparison with other STAs. For example, this could be based on themid-point between the spikes, or the “centre-of gravity” of thehistogram values.

For example, the chart shown in FIG. 7 shows an example type of signalthat could be transmitted by the interferer that is disregarding thequiet message, and is deliberately interfering. This example shows thetransmit power of an interfering signal initially at a first power for50% of the measurement time period, and then switching to a second(lower) power for the remaining 50% of the measurement time period.Other examples could use different ratios of time for the power levels,or have a lower power first, or have more than 2 power levels.

The table below illustrates a simplified example of how thisinterference can be interpreted by the RPI histogram. The rows representsampling periods of 100 μs at the measuring device, over a singlemeasurement time period. The right-most 8 columns show the differentpower “buckets” of the RPI histogram report. The second column from theleft shows the received interference power (in this example −77 dBm forthe first 5 sampling periods, then −87 dBm for the last 5 samplingperiods—i.e. reflective of the step function above).

Rx Time power RPI 0 RPI 1 RPI 2 RPI 3 RPI 4 RPI 5 RPI 6 RPI 7 (μs) (dBm)P ≤ −87 −87 < P ≤ −82 −82 < P ≤ −77 −77 < P ≤ −72 −72 < P ≤ −67 −67 < P≤ −62 −62 < P ≤ −57 −57 < P n −77 0 0 100 0 0 0 0 0 n + 100 −77 0 0 1000 0 0 0 0 n + 200 −77 0 0 100 0 0 0 0 0 n + 300 −77 0 0 100 0 0 0 0 0n + 400 −77 0 0 100 0 0 0 0 0 n + 500 −87 100 0 0 0 0 0 0 0 n + 600 −87100 0 0 0 0 0 0 0 n + 700 −87 100 0 0 0 0 0 0 0 n + 800 −87 100 0 0 0 00 0 0 n + 900 −87 100 0 0 0 0 0 0 0 RPI Density: 128 0 128 0 0 0 0 0

As can be seen, the RPI histogram firstly records 5 sample periods (eachof 100 μs) in RPI entry 2 (power bucket −82<P≤−77), and subsequently 5sample periods (each of 100 μs) in RPI entry 0 (power bucket P=−87). Thebottom row of the table shows an example of what may be seen in the RPIdensities in the final measurement report. The RPI density representsthe proportion of the overall measurement period that the received powerwas in each power bucket. This is represented in 8 bytes, with one byteper bucket and sums to approximately 255 (with some rounding errors)over all buckets. Therefore, if the received power was within one bucketfor the whole measurement period, then the RPI density for this bucketwould be 255 (the maximum representable in one byte) and the otherswould be zero. In the example above, the RPI density records zero ineach RPI entry, except a value of 128 for both entry 0 and 2,corresponding to a 50-50 split of interference power in these twobuckets. These are therefore the “spikes” in the histogram. A realsystem may not produce a report as clean as this, but there would stillbe detectable spikes. The spikes are of a matching size because the twopower levels were transmitted for the same duration, and the spacingbetween the spikes is dependent on the relative sizes of the powerlevels.

As an interferer gets closer or further away, the two spikes move leftor right in the histogram report. For example, a far-away (i.e. lowreceived power) interferer may give rise to an RPI report such as theone in the table above, as illustrated in FIG. 8a , in which the spikesindicate a low interference power. Conversely, a higher power (i.e.closer) interferer may give rise to an RPI report such as the one shownin FIG. 8b , in which the spikes indicate a high interference power.

The relative positions of the spikes for different interferers can beused as a measure of their relative distances away. For example, the“centre of gravity” of the RPI measurement report can be found and takenas the value used to compare interferers (e.g. this would obviously havethe RPI value 1 in the table above). This would be similar to findingthe mid-point between the two spikes in the absence of any noise orother interferers. However, any suitable comparison could be used (e.g.the position of the leftmost or rightmost spike).

The use of the two power levels (which causes the pair of spikes) givesa recognisable pattern that can be looked for in the histogram. It ispossible to have only a single power level and look for a single peak inthe histogram, but this may be more difficult to find in the presence ofnoise and other interferers. The two peak case is easier to find in areal system, as they are of a known height and spacing. Otherinterference issues could also be mitigated by performing measurementswithout the deliberate interference, and subtracting these from theresult. Alternatively, multiple measurements can be taken and averaged.

In another example, the interferer may have a ramp interference pattern,as illustrated in FIG. 9a . This example shows the transmit power of aninterfering signal ramping in a linear manner during the measurementperiod, t. In this case, the power starts at a non-zero value and isincreased. Other examples could ramp down, start at zero, etc.

FIG. 9b shows the received interference power, as measured by the devicethat is performing the standard Wi-Fi channel measurement. This chartshows four different received power profiles, each corresponding to theinterferer being a different distance away—profile 1 being the closest,and 4 being the furthest away.

Generally, as the interferer gets further away, the received powerprofile has a proportionately lower magnitude, but the slope of the rampstays broadly the same. Once the interferer is far enough away, some ofthe slope will disappear into the noise floor, as shown with profile 4.

By knowing the gradient of the ramp (the difference between the highestand lowest point), it can be determined how many buckets in thehistogram the power ramp would cover. Furthermore, for a linear ramp,approximately the same proportion of the measurement period should bespent in each of the buckets covered by the ramp, meaning that the RPIdensity is approximately equal for each of these buckets.

For example, for distance 1 in FIG. 9b , a strong interference is seen,implying the interferer is close by. This may result in the histogramshown in FIG. 10a . In this example, the ramp gradient is such that itcovers 5 histogram buckets, which are seen towards the upper end of thehistogram. The proportion of the measurement period with interference ineach of these 5 buckets (i.e. the RPI density) is x, which in thisexample can be expected to be approximately 255/5=51. FIG. 10b shows thehistogram for distance 2. The number of buckets with RPI density x isstill 5, as the gradient is the same, but has moved down the histogramdue the lower overall power. Similarly, the histogram for distance 3 inFIG. 10c shows the 5 buckets of x moving further down the histogram. Inthis case, the lowest received power is just at the noise floor, so theblock of 5 xs reach the lowest power bucket of the histogram. Fordistance 4, a portion of the ramp has disappeared under the noise floor.This results in a histogram, as shown in FIG. 10d , containing onlythree buckets with an RPI density of x in them, and the lowest buckethas an RPI density of 2x. This is because, in this example, the timespent below the noise floor is counted into the lowest bucket, resultingin twice as much time with an interference value in the lowest bucketrelative to the next three buckets.

The proximity of the interfering device can be estimated from the RPIhistogram based on where the recognisable block of RPI densities arelocated in the histogram (e.g. the trailing edge, leading edge etc). Theknowledge of what the RPI density should be in each bucket allow this tobe extended even when the interferer goes below the noise floor—e.g. inthe example described above it is known that there should be 5 bucketsof x, so it can be determined how much of the interference is not seen,and the proximity determined accordingly.

In a real system, the results may not be as clean as this, and noise andother interference may affect the results, with more inconsistent RPIdensities in the histogram. However, there would still be a recognisablepattern in the histogram. Other interference issues could also bemitigated by performing measurements without the deliberateinterference, and subtracting these from the result. Alternatively,multiple measurements can be taken and averaged.

In the case that a higher resolution was needed than could be capturedusing the 8-entry histogram, then multiple measurements could beperformed, with the interference ramp being split between themeasurements (e.g. measurement 1 ramps between power A and B,measurement 2 ramps between power B and C with the same gradient, etc.).

As described herein, the proximity estimate is based on one or moresignals being transmitted and received between one or more pairs ofdevices. Measured properties of the signal (such as signal strength, biterror rate, etc) can be indicative of a physical distance between thedevices. Generally, from the point of transmission of the signal, theintensity of the signal decreases with increasing distance. Thus,certain properties of the signal will worsen with increasing distance.By measuring certain properties of the signal at a receiving device itis possible to estimate how near or far the receiving device is from theposition that the signal was transmitted by the transmitting device.Thus, for example, a first measuring device that is a short distanceaway from a transmitting device can be considered to be more proximal tothe transmitting device than a second measuring device that is a greaterdistance away from the transmitting device.

Furthermore, the rate at which the intensity of the transmitted signaldecreases depends on the medium it is travelling through. For example,the signal can suffer from greater attenuation travelling throughobjects such as walls and ceilings/floors than travelling through air.Thus, in some cases, a transmitting device may be a short distance awayfrom the receiving device but a wall may greatly attenuate the signalbetween the two devices leading to a signal that is worse than if thesignal was travelling through only air. Thus, for example, a firstmeasuring device in the same room as the transmitting device may beconsidered to be more proximal to the transmitting device than a secondmeasuring device that is a shorter distance away but in another room asthe second measuring device will receive a worse signal due to greaterattenuation through a wall.

In the examples described herein, the received power of the transmittedsignal is utilised as a way of determining the strength of the receivedsignal to form the proximity measurements. However, the strength of thetransmitted signal may be determined in other ways, such as signalquality, data rates, bit error rates, number of acknowledgements, etc.For example, the signal transmitted by the transmitting device during aquiet interval may be a sequence of frames, from which the measuringdevice can determine a data rate. The data rate measured by themeasuring device can be reported to determine the strength of the signalreceived. The same sequence of frames may then be transmitted again sothat a different pair of transmitting/measuring devices (one of whichbeing the portable device) to determine another received data rate. Thereceived data rates can then be collated and analysed to determine theproximity of the portable device to the AP 10 and/or STAs 11 and/or 12.

Similarly, in another example, the transmitting device may transmit asignal (during a quiet interval) comprising a number of frames addressedto the measuring device. In accordance with a communications protocol(e.g. Wi-Fi) the transmitted frames may require acknowledgements to besent in response by the measuring device. The number of acknowledgementsreceived back by the transmitting device can indicate the strength ofthe signal received at the measuring device. Similarly to above, foranother pair of transmitting/measuring devices (one of which being theportable device), a similar signal may be transmitted with a similarnumber of frames addressed to the measuring device. The number ofacknowledgements received by each measuring device can be used to form ameasure of the proximity of the portable device to the AP 10 and/or STAs11 and/or 12.

As mentioned above, the general principles described herein can beapplied to devices and networks that operate according to communicationsprotocols other than Wi-Fi, such as Bluetooth.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

What is claimed is:
 1. A method of estimating the proximity of a firstdevice to a second device, the first and second devices capable ofcommunicating according to a wireless communications protocol, themethod comprising: at the first device, performing a channel qualitymeasurement during a first period of time; at the first device,receiving a signal from the second device during at least a portion ofthe first period of time, the received signal having a power modulatedaccording to a predefined pattern; and forming a measure of theproximity of the first device to the second device by analysing thepower of the received signal so as to detect the predefined pattern. 2.A method as claimed in claim 1, wherein said forming step comprisesanalysing a histogram so as to detect a representation of the predefinedpattern, wherein the histogram indicates received power at the firstdevice.
 3. A method as claimed in claim 2, wherein the wirelesscommunications protocol is an IEEE 802.11 protocol and the histogram isa received power indicator (RPI) histogram defined by the IEEE 802.11protocol.
 4. A method as claimed in claim 1, wherein the channel qualitymeasurement is, in accordance with the wireless communications protocol,for detecting interference.
 5. A method as claimed in claim 1, whereinthe wireless communications protocol is an IEEE 802.11 protocol and thechannel quality measurement is a measurement defined by the IEEE 802.11protocol.
 6. A method as claimed in claim 1, further comprising the stepof receiving a control message which, according to the wirelesscommunications protocol, instructs the first device to perform thechannel quality measurement during the first period of time.
 7. A methodas claimed in claim 1, further comprising the step of, at the firstdevice, performing another channel quality measurement during anotherperiod of time, wherein said forming step is additionally dependent onsaid another channel quality measurement.
 8. A method as claimed inclaim 1, further comprising the steps of: at the second device,receiving an instruction to not transmit during the first period of timeto enable the channel quality measurement to be performed by the firstdevice during the first period of time; and at the second device,disregarding said instruction and transmitting the signal during theportion of the first period of time.
 9. A first device for estimatingthe proximity of the first device to a second device, the first andsecond devices capable of communicating according to a wirelesscommunications protocol, the first device comprising: a controllerconfigured to cause the first device to perform a channel qualitymeasurement during a first period of time; a transceiver configured toreceive a signal from the second device during at least a portion of thefirst period of time, the received signal having a power modulatedaccording to a predefined pattern; and a proximity estimator configuredto form a measure of the proximity of the first device to the seconddevice by analysing the power of the received signal so as to detect thepredefined pattern.
 10. A device as claimed in claim 9, wherein theproximity estimator is configured to analyse a histogram so as to detecta representation of the predefined pattern, wherein the histogramindicates received power at the first device.
 11. A device as claimed inclaim 10, wherein the wireless communications protocol is an IEEE 802.11protocol and the histogram is a received power indicator (RPI) histogramdefined by the IEEE 802.11 protocol.
 12. A device as claimed in claim 9,wherein the channel quality measurement is, in accordance with thewireless communications protocol, for detecting interference.
 13. Adevice as claimed in claim 9, wherein the wireless communicationsprotocol is an IEEE 802.11 protocol and the channel quality measurementis a measurement defined by the IEEE 802.11 protocol.
 14. A device asclaimed in claim 9, wherein the transceiver is configured to receive acontrol message which, according to the wireless communicationsprotocol, instructs the first device to perform the channel qualitymeasurement during the first period of time.
 15. A device as claimed inclaim 9, wherein the controller is configured to cause the first deviceto perform another channel quality measurement during another period oftime, wherein the proximity estimator is configured to form the measureof the proximity in dependence on said another channel qualitymeasurement and the strength of the signal received at the first device.16. A non-transitory computer readable storage medium having storedthereon computer executable instructions that when executed, cause atleast one processor to: at a first device, perform a channel qualitymeasurement during a first period of time; at the first device, receivea signal from a second device according to a wireless communicationprotocol during at least a portion of the first period of time, thereceived signal having a power modulated according to a predefinedpattern; and form a measure of the proximity of the first device to thesecond device by analysing the power of the received signal so as todetect the predefined pattern.